Final answer:
Penelope's trick of undoing the shroud each night to delay marrying any of the suitors was discovered when a disloyal servant revealed her secret to them.
Step-by-step explanation:
Penelope's "shroud" trick stopped working because the servants she thought were loyal to her betrayed her. The shroud was intended for Laertes, Odysseus's father. Penelope promised the suitors that she would choose one of them when the shroud was completed.
However, every night, she undid part of the shroud's weaving, effectively never progressing. This allowed her to delay remarrying for three years.
Unfortunately, her ruse was eventually discovered when a disloyal servant informed the suitors of what Penelope had been doing, leading them to force her to complete it.